sell someone a pup
sell (one) a pup
To sell or pass off something to one that ultimately proves to be worth nothing or less than promised. I'm sorry, but that's definitely not an authentic Louis Vuitton bag. They sold you a pup, my friend. He told us he has years of experience in this field when we hired him, but he acts like this is his first time handling accounts like this. I'm starting to think he sold us a pup.
sell someone a pupswindle someone, especially by selling them something that is worthless. British informal
This phrase originated in the early 20th century; the idea behind it is presumably that of dishonestly selling someone a young and inexperienced dog when an older, trained animal had been expected.
1930 W. Somerset Maugham Cakes and Ale The public has been sold a pup too often to take unnecessary chances.